1. Field of the Invention
This invention relates to a portable video camera using stepping motors in automatic focusing and zooming mechanisms.
2. Description of the Related Art
Many portable video cameras, in particular small-scaled ones for home use, include built-in automatic focusing mechanism and zooming mechanism for easier video camera work. Automatic focusing and zooming mechanisms include lens drive means, and they are generally driven by stepping motors.
Stepping motors, in general, are essentially driven by a rectangular wave. However, stepping motors driven by a rectangular wave, in operation, generate noise caused by vibrations or higher harmonics. Therefore, most of stepping motors incorporated into video cameras are driven by a sine wave.
Driving by a sine wave is effected on the basis of data preliminarily stored in ROM without actually generating a sine wave. The ROM stores data indicating duty ratios of rectangular waves corresponding to sine waves taken by sampling at appropriate intervals. For example, the duty ratio is determined as 50% at angle 0 (rad), 100% at .pi./2, 0% at 3.pi./2, et seq. The stepping motor is driven by a rectangular wave generated on the basis of the data. At this time, the rectangular wave is smoothed into a sine wave by a bridge circuit for driving the stepping motor or by the coil of the motor itself.
Data stored in ROM corresponds to a sine wave. Therefore, the ROM need not store data of the full cycle from 0 to 2.pi., but may store, for example, data of only 1/4 cycle from 0 to .pi./2. When the stepping motor is driven, during 0 to .pi./2, data read out from ROM is used directly, and during .pi./2 to .pi., data stored in ROM is read out in the opposite sequence from that for 0 to .pi./2. During .pi. to 3.pi./2, data of 0 to .pi./2 stored in ROM is read out in the normal sequence but inverted in sign. During 3.pi./2 to 2.pi., data for 0 to .pi./2 is read out in the opposite sequence and assigned with the opposite sign. In this manner, by changing the order of reading data and/or changing the sign, where necessary, data of the full cycle from 0 to 2.pi. can be produced from data of 0 to .pi./2 alone.
When a stepping motor is driven by a sine wave, the cycle of the sine wave is modified to change the driving speed of the motor into a value for high-speed driving, low-speed driving, or the like. FIG. 7 shows sine waves of different cycles for different driving speeds. As illustrated, the cycle is short for high-speed driving and long for low-speed driving. The amplitude of the sine wave is constant regardless of the speed.
FIG. 8 shows a conventional circuit arrangement for driving a stepping motor. The stepping motor used here is of a PM (permanent magnet) type 2-phase-excited bipolar driving which is generally used for driving lenses of video cameras, floppy disk driving systems, and so forth.
MCU 100 supplies a read clock signal and an UP/DOWN signal to an address counter 101. Based on these signals, address data for reading out sine wave data from ROM 102 is supplied from the address counter 101 to ROM 102.
The read clock determines the frequency of the sine wave data read out from ROM 102. Therefore, if the intervals of the read clock are short, then the frequency of the sine wave is high, and the stepping motor is driven at a high speed. Similarly, if the intervals of the read clock are long, the stepping motor is driven at a low speed. The UP/DOWN signal inverts the sign of the sine wave data read out from ROM 102 from plus to minus, or vice versa, and the stepping motor is driven in the opposite direction.
ROM 102 also stores data on duty ratios of rectangular waves corresponding to sine waves of 1/4 cycle, i.e. from 0 to .pi./2. Therefore, when data is read out from ROM 102, the address counter 101 controls the order for reading out data in a predetermined cyclic range and adds the positive or negative sign to the data as explained above, thereby generates and outputs data for the full cycle from 0 to 2.pi..
Data on the duty ratios of rectangular waves corresponding to the sine waves is supplied to PWM generating circuits 103 and 104. The stepping motor to be driven here is of a 2-phase excitation type as stated above. Therefore, sine waves different in phase by .pi./2 are supplied simultaneously to the motor. Generation of waves in different phases can be realized by changing the way of reading data from ROM 102.
A rectangular-wave output from the PWM generating circuit 103 is supplied to the coil 107a of the stepping motor 107 via an H bridge circuit 105 for driving the stepping motor 107. A rectangular-wave output from the PWM generating circuit 104 is supplied to the coil 107b of the stepping motor 107 via an H bridge circuit 106. These rectangular-wave outputs are smoothed into sine waves (or cosine waves) by the H bridge circuit 105 and the coil 107a, or the H bridge circuit 106 and the coil 107b. Thus, driving of the stepping motor 107 by sinusoidal waves is realized.
FIG. 9 is a vector representation of the sine wave supplied to the stepping motor 107 driven in the above process. In the figure, arrows labelled phase A and phase B correspond to respective excitation vectors of two coils 107a and 107b. When sine and cosine waves are supplied to phase A and phase B, their composite vector draws a circle of a given diameter as illustrated.
In a video camera using such stepping motors, driving directions of the stepping motors are frequently changed for fine adjustment of focalization, or the like, during automatic focalization. Also during zooming function, rotating directions are frequently changed from one direction to the other. Moreover, if the camera includes the function of changing the zooming speed, a change in zooming speed causes a change in motor driving speed.
A large load is applied to a stepping motor under various conditions such as changes in driving direction, high-speed driving, starting of the motor, or low temperature, and if these conditions overlap, then the load to the stepping motor will become much larger. If, nevertheless, the driving voltage supplied to the stepping motor is not sufficiently large, then the motor cannot obtain a required torque, and produces a so-called power swing.
In a conventional system using a stepping motor of this type, the driving voltage (or driving current) for driving the stepping motor is set to a value with which a torque large enough to prevent power swing can be obtained even when a maximum load is applied to the stepping motor, among others, used in the system. Then, the same drive voltage (or drive current) is supplied also under a small load. The torque necessary for the motor driven under a small load may be smaller than that under the maximum load. Therefore, the conventional system fails to optimize the driving efficiency.
If a stepping motor of this type is used in a portable video camera normally relying upon a battery to be charged from time to time, useless consumption of battery power is a great disadvantage.